Supranuclear eye movements

1. SMOOTH PURSUIT SYSTEM

2. INTRODUCTION
• An object is seen with most detail when its image falls in the fovea.
• Two ocular motor systems allow visual images to remain in the fovea:
smooth pursuit( as the object moves vertically or horizontally)
• Vergence eye movements (convergence and divergence) as the object
moves along the depth axis of the visual field, particularly as it
approaches the subject.

3. • Images moving away from the fovea constitute the strongest stimuli
for smooth pursuit.
• Retinal slip is detected by the visual system and provides the
necessary stimulus for pursuit,a velocity error.
• Other stimulus for smooth pursuit is proprioceptive:subjects can
smoothly track their own outstretched finger when it is moving in
front of them in darkness.

4. THE PURPOSE OF SMOOTH PURSUIT
• Smooth-pursuit eye movements allow continuous, clear vision of
objects moving within the visual environment.
• Dodge demonstrated that the velocity of smooth-pursuit eye
movements matched the velocity of the target and it was continuous
in nature with no "periods of rest," and that vision remained clear
throughout the movement.
• Dodge's concept of smooth pursuit was eye movements that follow
moving targets.
• F.A. Miles has suggested that this system evolved to keep the fovea
pointed at a stationary target during self-motion.

5. • The smooth pursuit system cannot follow objects that move faster
than 30 to 40 degrees per second,
• the lower range being more characteristic of elderly persons.
• Faster moving objects elicit quicker eye movements, termed
saccades.
• Saccades are under the control of the will, but smooth eye
movements cannot be voluntarily produced and need a visual object
to be traced.

6. Pursuit Response to Nonvisual Stimuli
• Image motion on the retina is not the only stimulus capable of
eliciting smooth pursuit movements.
• Some subjects can smoothly track their own outstretched finger while
in darkness, probably using the consequent proprioceptive input
(reafference).
• Certain patients with acquired blindness can do the same.
• Few individuals can generate smooth eye movements without any
perception, or short-term memory, of a moving stimulus.

7. ANATOMY OF SMOOTH PURSUIT SYSTEM

9. • MT-speed and direction of the target
• Both MT and MST project via arcuate fiber bundles to the posterior
parietal cortex in the ventral bank of the intraparietal sulcus, which is
concerned with attention to moving objects

10. • The FEF generate volitional pursuit, which does not necessarily rely
on a moving visual stimulus (e.g.,predictive smooth pursuit or pursuit
of a moving target that transiently disappears).
• Area MST generates reflexive pursuit, in which a moving visual
stimulus is always required to elicit pursuit.
• Vertical pursuit signals follow a similar path and,after synapsing in the
vestibular nuclei, project rostrally through the MLF

11. Role of cerebellum
• The cerebellum plays an important role in synthesizing the pursuit
signal from visual and ocular motor inputs.
• The dorsal vermis and fastigial nucleus may contribute mainly to the
onset of pursuit,
• The parafloccuus and flocculus mainly sustain the pursuit response.

12. Accessory pathway of pursuit system
• Via the accessory optic system (AOS) and the nucleus of the optic
tract (NOT) .
• The AOS is composed of a group of midbrain nuclei that receives
inputs mainly from the contralateral retina via the accessory optic
tract.
• The NOT is a pretectal nucleus lying in the brachium of the superior
colliculus from which it receives retinal inputs.
• The AOS and NOT may play a role in activating the transcortical-
pontine cerebellar pursuit pathway.

13. QUANTITATIVE ASPECTS OF SMOOTH PURSUIT
• Smooth-pursuit performance varies considerably among individuals
• It is affected by many factors such as
• the properties of the stimulus, attention, and age.
• Smooth-pursuit eye movements are sensitive to the effects of many
medications

14. • Onset of Pursuit
• The initiation of smooth pursuit is most conveniently studied by
measuring eye position and velocity in the first second
• following presentation of a either a ramp or a step-ramp (Rashbass)
stimulus
• The latency to onset of smooth pursuit in response to a ramp target
motion is about 100 msec

16. • Initial Acceleration
• values for this initial eye acceleration are 40 to 100degree/sec,
varying from subject to subject.
• Thereafter, eye acceleration is a function of the velocity of image
motion on the retina and is decreased if the target is dimmer or if it
stimulates more peripheral retina.
• As target velocity is progressively increased, eye acceleration does not
increase by the same amount; this has been called an acceleration
saturation.

17. • Elderly subjects show a decrease in initial acceleration but no change
in the latency to onset of pursuit.
• Infants less than 12 weeks of age do show pursuit responses, but they
are small and intermittent.
• By 4 months of age, smooth pursuit responses improve and are more
related to target velocity

18. • Smooth-PursuitResponsesduring SustainedTracking
• Two types of stimuli are commonly used to test smooth pursuit: sine-wave
and constant- velocity movements
• Performance conventionally is evaluated by measuring gain (peak eye
velocity/peak target velocity) and phase. Phase is a measure of the
temporal synchrony between the target and the eye.
• During ideal pursuit tracking, the gain is close to 1.0 and the eye does not
lag behind the target (i.e., phase shift is small).
• Breakdown in smooth pursuit of a sinusoidal stimulus is indicated by a
decrease in gain and by the appearance of a phase lag of the eye with
respect to the target at higher frequencies.

20. CLINICAL EXAMINATION OF
SMOOTH PURSUIT
• Ask the patient to track a small target with the head still, such as a pencil
tip held a meter or more before the eyes.
• Initially, move the target at a low, uniform speed.
• Pursuit movements that do not match the target velocity necessitate
corrective saccades.
• If these are catch-up saccades,then the pursuit gain is low.
• If pursuit gain is too high (for example, because of superimposed slow
phases of nystagmus),then backup saccades are seen.
• During a series of regular to-and-fro movements of the test object,
suddenly stop the target motion at a turnaround point and look for a brief
continuation of pursuit; this tests the ability of the patient to use a
predictive strategy.

21. • Uncooperative or inattentive patients, small children, or those
thought to have hysterical blindness may be tested by slowly rotating
a mirror held before their eyes(a large mirror that fills most of the
visual field is a compelling stimulus for visual tracking.)
• Hand-held optokinetic drums or tapes do stimulate pursuit.

22. • For example, a patient with a right posterior cerebral lesion may show
fewer corrective quick phases when the drum is rotated to the right
side.
• This is in part because the pursuit gain is lower to the right, and,
because the eyes deviate more slowly from the primary
position,fewer quick phases are needed.

23. LABORATORY EVALUATION
OF SMOOTH PURSUIT
• A prerequisite for smooth-pursuit testing is to maintain the alertness
and attention of the subject or patient;
• recording sessions should be kept as short as possible.
• The most commonly used stimulus for smooth pursuit is a small,
bright spot of light, typically from a Helium-Neon laser, projected
onto a dark or featureless screen.
• An alternative to a projected stimulus is a bright spot on a video
screen

24. Abnormalities of Smooth Pursuit
• Abnormalities of initiation, gain, and symmetry
• INITIATION:
• Unilateral lesions of striate cortex cause a loss of smooth pursuit for
targets moving in the blind visual field;
• this deficit is not evident during pursuit of a predictably moving target
partly because of preserved macular vision.
• In humans, bilateral occipital lobe lesions abolish or prevent the
development of smooth pursuit.

25. • Low gain pursuit is a common ocular motor abnormality.
• Low gain occurs with a variety of conditions, including old
age,Parkinson's disease,progressive supranuclear palsy, and following
large cerebral lesions.
• Impairment of smooth pursuit due to an abnormal acceleration
saturation is seen with posterior cortical lesions,Alzheimer's
disease,and schizophrenia.

26. • Excessively high pursuit gain may reflect adaptive changes due to
extraocular muscle palsy.
• If, for example, a patient with partial left abducens palsy is forced to
use that eye (by patching the normal, right eye), increased
innervation is sent to the weak muscle.
• If, after several days, the patch is switched so that the normal eye
views again, smooth-pursuit gain of the right eye to the left is
increased and tracking is unstable with pendular oscillations.

27. • THANK YOU